The present invention relates to a detection system, and particularly though not exclusively to a detection system suitable for detecting circular dichroism.
Circular dichroism is the differential absorption between left and right circularly polarised light on passage through a sample. When light passes through a sample, linear absorbance of the light occurs with the result that the amount of light that passes from the sample is less than the amount of light that passed into the sample. Measuring this difference provides a measurement of the linear absorbance of the sample. When the light is circularly polarised a secondary absorbance component arises from circular dichroism. The secondary absorbance component is measured by switching between left and right circularly polarised light, and measuring the resulting difference in absorbance.
Measurement of circular dichroism gives detailed structural and enantiomeric (handedness) information on proteins, carbohydrates, nucleic acids, pharmaceuticals, liquid crystals, etc. With circular dichroism one can follow e.g. the conversion of simple peptides into the destructive fibrils of CJD, Alzheimer's, cystic fibrosis etc. Handedness was at the heart of the thalidomide disaster.
Since circular dichroism is a secondary component of the measurement of absorbance, it is a difficult property to measure. Typically the absorbance due to circular dichroism is around one part in 105 of the mean intensity of light transmitted by a sample. Measurement is further complicated by the fact that absorbance measurements are often performed at ultraviolet and deep ultraviolet wavelengths (i.e. <200 nm).
It is conventional to detect circular dichroism by modulating the polarisation of light incident on a sample using a polarising modulator, and then detecting the modulation of light transmitted by the sample using phase-locked detection. The polarising modulator is for example configured to switch the polarisation of the incident light beam between left hand polarisation and right hand polarisation at a frequency of 50 kHz, and the phase-locked detector measures at 50 kHz light incident upon the detector. This allows detection of circular dichroism which effectively comprises a small AC signal on a very large DC background.
Conventionally, a single element detection system is used to detect circular dichroism. The system comprises a photo multiplier tube with a high dynamic range (typically 109), coupled to a phase-locked signal extraction amplifier that is able to distinguish the circular dichroism signal from the background signal. The detection system uses a servo system to adjust the high tension on the photo multiplier to produce a constant DC current output, independent of the intensity of DC light incident upon the photo multiplier tube. This is done to compensate for changes of beam intensity and of linear absorbance as the wavelength of the incident light is scanned (the beam intensity and linear absorbance may change by four orders of magnitude). The constant DC current output is advantageous because it provides a constant DC level from which the AC circular dichroism signal may be easily phase-lock extracted.
The wavelength range of interest is scanned wavelength by wavelength (with about 1 nm resolution). The scanning process takes many minutes when the circular dichroism signal is strong, but many hours when the circular dichroism signal is weaker (as is the case for most useful circular dichroism work).
It is an object of the present invention to provide a detection system which overcomes or mitigates at least one of the above disadvantages.
According to a first aspect of the invention there is provided a detection system comprising modulation means for applying a modulation to an incident beam of radiation, sample holding means through which the modulated beam of radiation is passed, beam expansion means to expand the beam of radiation, an array of solid state detectors arranged to receive different parts of the expanded beam of radiation, and processing means arranged to synchronise detected signals with the modulation applied by the modulation means.
The detection system is advantageous because the use of an array of solid state detectors, rather than the single detector used by the prior art, allows more information to be determined from the detected signals.
Preferably, the processing means further comprises amplification means to amplify signals detected by the array of solid state detectors.
The detectors are preferably solid state detectors although other forms of detector, such as for instance multi-channel photo-multipliers could be used.
Preferably, the processing means further comprises digitisation means to digitise detected signals.
Preferably, the processing means is arranged to digitise detected signals before they are synchronised with the modulation applied by the modulation means. This is advantageous as compared to the conventional approach of synchronising before digitisation, since it provides faster and more sensitive measurement.
The beam expansion means may comprise wavelength separation means arranged to convert the beam of radiation into a diverging fan of wavelengths.
Preferably, the wavelength separation means comprises a reflective grating.
Preferably, the system further comprises a steering mirror, the orientation of the steering mirror being adjustable to allow selection of the wavelengths of the fan that are incident upon the array of solid state detectors.
The beam expansion means may comprise means for expanding the beam of radiation whilst retaining spatial properties of the beam in at least one direction perpendicular to the direction of propagation of the beam of radiation.
Preferably, the array of solid state detectors is a two dimensional array.
Wavelength separation may be provided in a first direction perpendicular to the direction of propagation of the beam of radiation, and beam expansion whilst retaining spatial properties may be provided in a second direction perpendicular to the direction of propagation of the beam of radiation, the two dimensional array being used to detect wavelength dependent properties and spatial properties of the expanded beam.
Preferably, the array of solid state detectors comprises a first array of photodiodes arranged to detect near ultraviolet wavelengths, and a second array of photodiodes arranged to detect deep ultraviolet wavelengths.
Preferably, the first array of photodiodes comprises silicon.
Preferably, the second array of photodiodes comprises AlGaN.
Preferably, the modulation means is configured to apply a modulated circular polarisation to the beam of radiation. It will be appreciated that other forms of modulation may be applied to the beam of radiation.
Preferably, the modulation means is configured to apply a modulation at a frequency greater than 1 kHz.
Preferably, the sample holding means is provided with an adjustable aperture, which allows adjustment of the width of beam incident upon a sample.
Preferably, each detector of the array of solid state detectors is provided with a transconductance amplifier arranged to convert photocurrent output by the detector into a voltage.
Preferably, the transconductance amplifier is provided with a plurality of resistors which may be connected to the transconductance amplifier in different combinations using a switch, to modify the gain of the transconductance amplifier.
Preferably, the switch is controlled by a microprocessor.
Preferably, each detector of the array of solid state detectors is provided with an AC amplifier arranged to amplify AC components of a signal detected by the solid state detector.
Preferably, the AC amplifier is provided with a plurality of resistors which may be connected to the transconductance amplifier in different combinations using a switch, to modify the gain of the AC amplifier.
Preferably, the switch is controlled by a microprocessor.
Preferably, the system is provided with a band pass filter tuned to the frequency of operation of the modulation means.
Preferably, the system further comprises a multiplexor arranged to multiplex, for each detector of the array of solid state detectors, a DC signal and an AC signal after amplification.
Preferably, synchronisation is performed by a field programmable array. The skilled person will appreciate that a field programmable array (FPGA) is a known form of commercially available programmable electronic circuit which may be integrated on a single chip with very fast operation and small size. A single chip may contain very large numbers (in excess of 20,000) semi conductor gates that can be configured to form descrete electronic function circuits, such as adders and the like. The circuit can then be programmed to perform arithmetic operations, such as addition, subtraction and division, very rapidly. The ability to re-configure such arrays is an advantage over existing ASIC technology.
Preferably, the system further comprises a personal computer arranged to receive data, and to control operation of components of the system.
Preferably, the array of solid state detectors is translatable and pivotable to allow it to be aligned with the beam.
According to a second aspect of the invention there is provided a detection system comprising modulation means for applying a modulation to an incident beam of radiation, sample holding means through which the modulated beam is passed, at least one solid state detector, and processing means arranged to amplify and digitise signals detected by the at least one detector, and then to subsequently synchronise the amplified and digitised signal with the modulation applied by the modulation means.
The second aspect of the invention is advantageous because it provides detection which is faster and more sensitive than conventional circular dichroism measurement systems, where the signal is synchronised with the polarising modulator before it is digitised.
The second aspect of the invention may further comprise any of the preferred features of the first aspect of the invention.
According to a third aspect of the invention there is provided a detection method comprising applying a modulation to an incident beam of radiation, passing the modulated beam of radiation through a sample held in a sample holding means, expanding the beam of radiation, detecting different part of the expanded beam of radiation using an array of solid state detectors, synchronising detected signals with the applied modulation.
Preferably, the detected signals are amplified.
Preferably, the detected signals are digitised.
Preferably, the detected signals are digitised before they are synchronised with the applied modulation.
According to a fourth aspect of the invention there is provided a detection method comprising applying a modulation to an incident beam of radiation, passing the modulated beam of radiation through a sample, detecting the beam of radiation using at least one solid state detector, amplifying and digitising the signals detected by the at least one detector, and then subsequently synchronising the amplified and digitised signals with the applied modulation.
The fourth aspect of the invention may further comprise any of the preferred features of the second aspect of the invention.